Think about it real long and hard and carefully. Are you really sure and comfortable with the idea of her having sex with another man? It could doom your relationship even if you think right now that it's o.k. If she really enjoys the size of a much larger man, then she might at times want it again. Will you be o.k with that? Knowing that she doesn't feel truly satisfied by you?
Also she might develop an emotional bond to the guy. Where would that leave you?
If she is curious as to what a large penis feels like, have her take the first step of getting a nice life like dildo maybe even supply you with a strap on extension so it'll be as close as possible to you actually making love to her with a big penis.
Don't rush into it.
Or how about dump her? In my experience, women don't respect artificial stuff like penis extension... She clearly wants the real thing. Let her explore, meet me her later in life if that's possible